1. Field of the Invention
The present invention relates to methods and compositions for drilling wellbores in subterranean formations using aqueous based drilling fluids. More particularly, the present invention relates to methods for drilling depleted sands, especially depleted sands having several hundred pounds of overbalance pressure per square inch of subterranean formation, and methods for treating such sands to reduce loss of drilling fluid during such drilling.
2. Description of Relevant Art
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drillbit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain integrity of the wellbore walls and to prevent wellbore blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation. Because of the diversity of geological formations encountered in the oil and gas industry, drilling fluids usually are custom-blended to provide the specific rheology and other properties required for each situation. Generally, drilling fluid systems are complex compositions of liquids (i.e., water, petroleum oil, or synthetic oil or other synthetic fluid), dissolved inorganic and/or organic additives, and suspended, finely divided solids of various types.
Formation damage due to invasion by drilling fluids is a well known problem in drilling. Invasion of drilling fluids into the formation is caused by the differential pressure of the hydrostatic column which is generally greater than the formation pressure, especially in low pressure or depleted zones. Invasion is also caused or allowed by openings in the rock and the ability of fluids to move through the rock—the porosity and permeability of the zone. When drilling depleted sands under an overbalance condition, which may be several hundreds of pounds per square inch, mud pressure will penetrate progressively into the formation unless there is an effective flow barrier present at the wellbore wall. Horizontal drilling may also drill across highly fractured or permeable, low pressure or depleted zones, which increases the probability of the drill pipe getting stuck due to lying on the low side of the borehole. The exposure of numerous fractures or openings having low formation pressures has increased the problems of lost circulation and formation invasion.
Filtrate control mechanisms have long been used to control the movement of drilling fluids and filtrate into and through the formation openings. Such mechanisms traditionally involve adding particles to the drilling fluid, which are deposited onto the wellbore wall during circulation of the drilling fluid when drilling. These particles are commonly bentonite, starch, lignins, polymers, barite, drilled solids and various combinations of these. The particles form a filter cake on the borehole wall which provides a low permeable barrier. Such traditional solutions, however, have not been sufficient for eliminating or significantly reducing lost circulation and formation invasion of drilling fluids when drilling depleted sands, particularly depleted sands with overburden pressure amounting to several hundred pounds per square inch, or across highly fractured or permeable, low pressure or depleted zones. There continues to be a need for improved methods for drilling depleted sands.